Re: [PATCH 1/2 v2] hwrng: Allow drivers to disable reading during probe

[Kees Cook]

On Wed, Jul 2, 2014 at 6:41 AM, Jason Cooper <jason@xxxxxxxxxxxxxx> wrote:
> On Wed, Jul 02, 2014 at 06:56:35PM +0530, Amit Shah wrote:
>> Hi Jason,
>>
>> On (Wed) 02 Jul 2014 [13:00:19], Jason Cooper wrote:
>> > Commit d9e7972619334 "hwrng: add randomness to system from rng sources"
>> > added a call to rng_get_data() from the hwrng_register() function.
>> > However, some rng devices need initialization before data can be read
>> > from them.
>> >
>> > Also, the virtio-rng device does not behave properly when this call is
>> > made in its probe() routine - the virtio core sets the DRIVER_OK status
>> > bit only on a successful probe, which means the host ignores all
>> > communication from the guest, and the guest insmod or boot process just
>> > sits there doing nothing.
>> >
>> > [ jac: Modify the API to allow drivers to disable reading at probe, new
>> > patch, copied Amit's commit message. ]
>> >
>> > CC: Kees Cook <keescook@xxxxxxxxxxxx>
>> > CC: Jason Cooper <jason@xxxxxxxxxxxxxx>
>> > CC: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
>> > CC: <stable@xxxxxxxxxxxxxxx> # v3.15+
>> > Signed-off-by: Amit Shah <amit.shah@xxxxxxxxxx>
>> > Signed-off-by: Jason Cooper <jason@xxxxxxxxxxxxxx>
>> > ---
>> >  drivers/char/hw_random/core.c | 8 +++++---
>> >  include/linux/hw_random.h     | 4 ++++
>> >  2 files changed, 9 insertions(+), 3 deletions(-)
>> >
>> > diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
>> > index 334601cc81cf..b7b6c48ca682 100644
>> > --- a/drivers/char/hw_random/core.c
>> > +++ b/drivers/char/hw_random/core.c
>> > @@ -347,9 +347,11 @@ int hwrng_register(struct hwrng *rng)
>> >     INIT_LIST_HEAD(&rng->list);
>> >     list_add_tail(&rng->list, &rng_list);
>> >
>> > -   bytes_read = rng_get_data(rng, bytes, sizeof(bytes), 1);
>> > -   if (bytes_read > 0)
>> > -           add_device_randomness(bytes, bytes_read);
>> > +   if (!(rng->flags & HWRNG_NO_READ_AT_PROBE)) {
>> > +           bytes_read = rng_get_data(rng, bytes, sizeof(bytes), 1);
>> > +           if (bytes_read > 0)
>> > +                   add_device_randomness(bytes, bytes_read);
>> > +   }
>>
>> But this has the inverse problem: if there are two hwrngs in the
>> system, one will be initialized and probed.  The other will not be
>> initialized, but still be probed.
>
> That's a problem outside the scope of this patch.  You're basically
> saying the ->init() should be called unconditionally for each hwrng.  If
> that's what driver authors assumed, that's not what is happening if
> there is more than one driver in the system.

My intent with the patch was to get randomness added when a hwrng was
available. Perhaps instead of skipping this call, it can be done
either during probe (for devices that support it or lack an init
function), or done after initialization?

> I think you should be changing the code a few lines up to make sure
> hwrng_init() is called once for each driver.

Is that really how it should work? I'd be for the "always-init"
approach since it gains us access to the entropy, but I have to play
devil's advocate: would one expect "probe" to just probe, not
initialize? That seems more like a function of enabling the device.

-Kees

-- 
Kees Cook
Chrome OS Security
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